This invention relates to an apparatus and method for controlling the transmission of light of a selected wavelength through a material having a thermally sensitive coefficient of optical absorption. More particularly, this invention relates to optical switching devices in which the transmissivity of the device for a selected wavelength of light is switched from a more transmissive state to a less transmissive state (or from a less transmissive state to a more transmissive state) by application of one or more control signals to the device to activate a thermal switching mechanism. The control signals are independent of the light being switched.
Switching is a basic function in the developing fields of optical signal processing and communications. As these fields have developed, and as optical signal processing and communication systems have become more complex and more widely used, a need has arisen for switching devices that are versatile, efficient and easily fabricated.
One proposed form of an optical switch is the generalized cross bar switch. Such a switch comprises, in theory, a matrix of optical switching devices each of which can be independently controlled to switch a separate light signal. An optical cross bar switch has many potential uses. In fiber optic communications, for example, an optical cross bar switch can be used to provide a link between one bundle of optical fibers and another. In optical signal computing, an optical cross bar switch can be used to interconnect optical computing elements.
An optical cross bar switch having a high density of switching devices would be particularly useful in performing massively parallel processing operations. For example, a high density cross bar switch can be used as a spatial light modulator for parallel processing of two-dimensional data-like images. Preferably, the modulator would be implemented such that the data signal impinges orthogonally on the modulator surface. This would be an improvement over known "waveguide" structures which require light signals to travel parallel to the surface of the structure.
The above-mentioned "waveguide" structures are but one of several technologies that have been or are being investigated in connection with optical switching. Several such technologies are based on optical properties of semiconductor materials. For example, semiconductor structures known as multiple quantum well (MQW) structures, which include devices known as self-electro-optic devices (SEEDs), have been investigated as a potential technology for implementing optical switching devices. Semiconductor laser amplifiers, many of which fall into the "waveguide" category in that they accept light parallel to the semiconductor wafer, have also been considered as potential switching devices. These known semiconductor technologies suffer various drawbacks when used to implement optical switching devices, including in different cases one or more of the following: low on/off contrast ratios, high power or high biasing voltage requirements, high noise levels and technological limits on the growth of the structures.
Several different non-semiconductor technologies also have been investigated as possible solutions for developing optical switching devices. These technologies include switches based on liquid crystals, lead lanthanum, zirconiuim titanite (also known as PLZT), and LiNbO.sub.3. Such switches, however, also suffer various drawbacks. Many require polarized light and have high insertion losses. High voltage requirements and low on/off contrast ratios are also problems in many cases. In addition, techniques for fabricating such devices in high densities are not readily available, in contrast to the high density, integrated circuit processing techniques that are readily available for semiconductor-based devices.
To illustrate some of the principles employed in the prior art to achieve optical switching, and to more particularly illustrate some of the problems encountered by prior art techniques, a discussion follows of two such techniques.
It is known in the prior art that a semiconductor material will absorb incident photons having energies equal to or greater than the band gap energy of the material, and will transmit incident photons having energies less than the band gap energy of the material. The degree to which a semiconductor material will absorb an incident light signal of a particular photon energy (or wavelength) is characterized by an absorption coefficient. Generally, the absorption coefficient of a semiconductor material varies non-linearly over the photon energy spectrum, with a particularly high rate of change near the edge of the semiconductor band gap.
It is also known that the band gap energy of a semiconductor material can be caused to shift by exciting the material. For example, it is known that by heating the semiconductor material its band gap energy can be caused to decrease. On the other hand, it is known that population inversion excitation (for laser-type amplification) can be used to cause the band gap energy to shift in the opposite direction.
These properties of semiconductors have been used in at least one known instance to develop an experimental structure which can be caused to switch between a transmissive state and an absorptive state depending on the power of an incident light signal. This structure is described in D. A. B. Miller, A. C. Gossard and W. Wiegmann, "Optical bistability due to increasing absorption", Optics Letters, Vol. 9, No. 5, May 1984, pp. 162-4.
According to the Miller et al. article, switching results in the described structure because, at photon energies near the band gap energy of the semiconductor material of the structure, absorption increases as a non-linear function of the excitement of the material. Any absorption causes the temperature of the semiconductor material to increase. As a result of the increase in temperature, the band gap energy of the semiconductor material decreases and the absorption increases, thus producing a regenerative feedback mechanism. If the power of the incident light signal is increased to a certain threshold level, theoretically this regenerative feedback mechanism causes the optical absorption to increase dramatically and switches the structure into a low transmission state.
This switching methodology, however, suffers several drawbacks. For one, the switched light signal's amplitude controls switching. Therefore, switching control is not independent of the light being switched--complicating and restricting applications. Moreover, the switching mechanism is highly sensitive to the wavelength of the switched light signal.
The non-linear absorption characteristics of semiconductors also have been investigated using a laser beam of variable power to excite (population invert) a semiconductor material while a second light signal is incident on the material. See Y. H. Lee et al., "Room-Temperature Optical Nonlinearities in GaAs," Phys. Rev. Letters 57, 2446 (1986). As a possible method for optical switching, however, this laser amplifier excitation technique described in the article suffers the drawbacks of high noise generation and of requiring a high power input density to switch. Furthermore, the laser amplifier switching mechanism is not thermal.
In view of the foregoing, it would be desirable to be able to provide an optical switching device that can be optically controlled in a manner that does not depend on the amplitude or other properties of the light signal being switched; that can be electrically controlled at low voltage; that is not particularly sensitive to the wavelength of the light being switched; that can achieve a high on/off contrast ratio and low insertion loss; that has low noise; and that can be manufactured easily using conventional integrated circuit processing techniques.
It would also be desirable to be able to provide an optical switching device that can be fabricated as part of a high density matrix of such devices to form an optical cross bar switch for use in applications such as fiber optic communication and optical signal processing.